[gesh]

💡 Motivation and Context

• MCP clients list tools on every reconnect, so $mcp_tools_list is about as common as $mcp_tool_call.
• On PostHog's own MCP server, it was 30% of all MCP events in the last 7 days: 17.4M events, against 17.2M tool calls.
• Hosts that do not use the event pay for it and cannot turn it off.

Follows #1115, which added the same option for $mcp_initialize. Part of one change across three repos: PostHog/posthog-js#5404 and PostHog/sdk-specs#142.

Changes

• New option MCPAnalyticsOptions(send_tools_list_event=False). It is on by default, so nothing changes unless a host sets it.
• The name says "event", like send_initialize_event in feat(mcp): add a send_initialize_event option #1115. PostHogMCP.capture_tools_list() already captures the event by hand.
• With the option off, record_tools_list returns early. It is the only place that sends the event, so a failed listing's $exception goes too.
• Nothing else changes. Tools keep their injected arguments, and the virtual tools stay listed.
• README line and a brief minor changeset.

💚 How did you test it?

• test_send_tools_list_event_controls_only_the_listing_event runs with the option on and off over HTTP. It covers the SDK high-level server, standalone fastmcp, and a low-level Server, on both SDK majors. A listing sends one or no $mcp_tools_list. The listed schema still has the injected context, and the tool call still records its intent.
• test_send_tools_list_event_off_also_drops_a_failed_listing checks that a failed listing sends no event and no $exception (SDK 1.x).
• Without the guard, the four option-off cases fail.
• The MCP suites pass on Python 3.10 and 3.14. SDK 1.x has 657 tests, and SDK 2.x has 548.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Written with Claude Code, model Claude Opus 5.5 (claude-opus-5-5).
• The option only stops the event. The tools/list wrapper still injects arguments, because tool calls depend on them.

🤖 Generated with Claude Code

[github-actions]

posthog-python Compliance Report

Date: 2026-10-28T13:51:06.362799+00:00
Duration: 259841ms

✅ All Tests Passed!

121/121 tests passed

---

Capture_V1 Tests

✅ 95/95 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	517ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	512ms
Required Headers.Has Authorization Bearer Header	✅	510ms
Required Headers.Has Content Type Json	✅	510ms
Required Headers.Has Posthog Sdk Info Format	✅	511ms
Required Headers.Has Posthog Attempt Header	✅	510ms
Required Headers.Has Posthog Request Id	✅	511ms
Required Headers.Has Posthog Request Timestamp	✅	510ms
Required Headers.Has User Agent	✅	511ms
Body Format.Body Has Created At And Batch	✅	510ms
Body Format.No Api Key In Body	✅	510ms
Body Format.No Sent At In Body	✅	510ms
Event Format.Event Has Required Root Fields	✅	510ms
Event Format.Event Uuid Is Valid	✅	511ms
Event Format.Event Timestamp Is Rfc3339	✅	510ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	525ms
Event Format.Distinct Id Is String	✅	510ms
Event Format.Distinct Id At Root Not Properties	✅	510ms
Event Format.Custom Properties Preserved	✅	510ms
Event Format.Set Properties Preserved	✅	511ms
Event Format.Set Once Properties Preserved	✅	510ms
Event Format.Groups Properties Preserved	✅	510ms
Event Format.Sdk Generates Uuid If Not Provided	✅	511ms
Event Format.Event Has Required Root Fields Batch	✅	513ms
Event Format.Event Uuid Is Valid Batch	✅	514ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	515ms
Event Format.Distinct Id Is String Batch	✅	514ms
Event Format.Distinct Id At Root Not Properties Batch	✅	514ms
Event Format.Custom Properties Preserved Batch	✅	513ms
Event Format.Set Properties Preserved Batch	✅	514ms
Event Format.Set Once Properties Preserved Batch	✅	514ms
Event Format.Groups Properties Preserved Batch	✅	514ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	514ms
Batch Behavior.Multiple Events In Single Batch	✅	518ms
Batch Behavior.Batch Envelope Smoke	✅	516ms
Batch Behavior.Flush With No Events Sends Nothing	✅	506ms
Batch Behavior.Flush At Triggers Batch	✅	1011ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	511ms
Deduplication.Generates Unique Uuids	✅	518ms
Deduplication.Different Events Same Content Different Uuids	✅	513ms
Deduplication.Preserves Uuid On Retry	✅	6517ms
Deduplication.Preserves Timestamp On Retry	✅	6520ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6524ms
Deduplication.No Duplicate Events In Batch	✅	518ms
Header Behavior On Retry.Attempt Header Starts At One	✅	510ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13529ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6520ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3019ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6516ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	510ms
Response Format Validation.Success Response Has Ok For Each Event	✅	515ms
Response Format Validation.Success No Retry After When All Ok	✅	513ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1516ms
Response Format Validation.Success No Retry After When Drop Only	✅	513ms
Response Format Validation.Response Echoes Request Id	✅	511ms
Retry Behavior.Retries On 408	✅	6515ms
Retry Behavior.Retries On 500	✅	6520ms
Retry Behavior.Retries On 503	✅	8524ms
Retry Behavior.Retries On 504	✅	6520ms
Retry Behavior.Retryable Errors Have Retry After	✅	3517ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11517ms
Retry Behavior.Does Not Retry On 400	✅	2514ms
Retry Behavior.Does Not Retry On 401	✅	2513ms
Retry Behavior.Does Not Retry On 402	✅	2514ms
Retry Behavior.Does Not Retry On 413	✅	2514ms
Retry Behavior.Does Not Retry On 415	✅	2514ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2514ms
Retry Behavior.Implements Backoff	✅	22537ms
Retry Behavior.Max Retries Respected	✅	22534ms
Partial Batch Handling.Handles 200 Full Success	✅	2513ms
Partial Batch Handling.Handles 200 With All Ok	✅	3515ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3514ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3516ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6522ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6518ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6519ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6521ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6521ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6517ms
Partial Batch Handling.Respects Retry After On Partial	✅	8519ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3514ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3517ms
Compression.Sends Gzip Content Encoding	✅	511ms
Compression.No Content Encoding When Disabled	✅	511ms
Compression.Compressed Body Is Decompressible	✅	510ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2512ms
Event Options.Cookieless Mode Override	✅	511ms
Event Options.Disable Skew Correction Override	✅	511ms
Event Options.Process Person Profile Override	✅	510ms
Event Options.Product Tour Id Override	✅	510ms
Event Options.Unset Options Omitted	✅	511ms
Event Options.Options Override In Batch	✅	514ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	510ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	510ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	510ms

Capture_Ai Tests

✅ 5/5 tests passed

View Details

Test	Status	Duration
Routing.Capture Ai Posts To Ai Endpoint	✅	511ms
Routing.Capture Does Not Reroute Ai Named Events	✅	510ms
Identity.Capture Ai Event Has Uuid	✅	510ms
Identity.Capture Ai Keeps Supplied Uuid	✅	510ms
Timestamp Format.Non Utc Event Timestamp Is Converted To Utc	✅	510ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	11ms
Request Payload.Flags Request Uses V2 Query Param	✅	9ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	9ms
Request Payload.Flags Request Omits Authorization Header	✅	9ms
Request Payload.Token In Flags Body Matches Init	✅	9ms
Request Payload.Groups Round Trip	✅	9ms
Request Payload.Groups Default To Empty Object	✅	9ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	10ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	9ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	9ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	509ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	15ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	11ms
Retry Behavior.Retries Flags On 502	✅	314ms
Retry Behavior.Retries Flags On 504	✅	316ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	512ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	70ms
Versioned Boolean Matching.Matching Version 1	✅	67ms
Versioned Boolean Matching.Matching Version 2	✅	67ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	35ms

[gesh]

@PostHog review

[posthog]

@gesh PostHog Review is reviewing this pull request.

[posthog]

PostHog Review · Standard · reviewed 65e9ae3: no findings.

[stamphog]

Not approved — this change needs a human reviewer.

Re-adding the stamphog label gives the same result unless the changed files change.

Stamphog can't approve this pull request automatically. The deny-list gate refused it because posthog/mcp/types.py and references/public_api_snapshot.txt change the public API by adding the capture_tools_list option to MCPAnalyticsOptions. The tier gate also classified the change as T2-never (cross-cutting feature, 85 lines across 6 files), which is never eligible for automated approval. The size gate passed, so splitting the pull request won't change this outcome. Please ask a human reviewer to look at it, ideally one who owns the MCP public API.

• posthog[bot] reviewed the current head.

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✗	matches: public_api
size	✓	6L, 2F substantive, 85L/6F incl. docs/generated/snapshots — within ceiling
tier	✗	classified as T2-never: T2-never (85L, 6F, cross-cutting, feat)
stamphog 2.6.1		.stamphog/policy.yml @ unknown · reviewed head 65e9ae3

[lucasheriques]

LGTM, approving. The naming question on #1115 applies to capture_tools_list too.

[gesh]

@lucasheriques renamed capture_tools_list to send_tools_list_event in 641fa39, to match #1115.